Device and method for reducing thermal cycling in a semiconductor package

ABSTRACT

In semiconductor packing, a method and device for reducing thermal stress on a die and for reinforcing the strength of a die, A thermally-conductive member is positioned in a cooperating manner with the die during the packaging process.

FIELD OF THE INVENTION

This invention relates generally to semiconductor packaging and moreparticularly to a device and method for reducing the thermal stress onan electrically-functional die and re-enforcing the strength of the diepositioned within a package.

BACKGROUND OF THE INVENTION

A semiconductor package is used to house an integrated circuit (IC) chipto protect it and provide it with electrical connectors for attachmentto an printed circuit (PC) board. As shown in FIG. 1, the IC chip or die11 is positioned on top of a die attachment pad (DAP) 12 which is housedin a package 13, typically made of plastic. The die 11 is electricallyconnected to an PC board (not shown) by wires 14 which are connected toleads 16. The die 11 is attached to the DAP 12 by adhesive material 17such as epoxy or soft solder.

During board assembly of electronic or computer products, each lead andIC board electrical connection is individually soldered. However, insurface mount technology, the IC chip's and board's electricalconnections are formed by baking the entire unit in an oven at atemperature of approximately 230° C. The unit is then cooled to roomtemperature. The screening tests for military applications includestemperature extremes from -60° C. to 150° C. An electrically conductiveadhesive material bonds the leads of the semiconductor package to theboard. Surface mount technology provides a finished product of greatlyreduced size and requires significantly less labor in its manufacturethan one produced by conventional technology.

During processing the board and packaged IC chips, are thereforesubjected to temperature extremes totalling approximately 230° C. Thesetemperatures subject the IC chip contents of the semiconductor packageto thermal stress which can damage and reduce the structural integrityof the IC chip.

The following table lists the coefficients of thermal expansion of thecomponents of a semiconductor package.

    ______________________________________                                        Selected Packaging Materials Properties                                                 Coefficient of                                                                Thermal Expansion                                                                            k(W/                                                 Material  CTE(× 10.sup.-6 /°C.)                                                           m.°K.)                                                                         E(× 10.sup.6 psi)                      ______________________________________                                        Plastic Package                                                                         30-85          0.837   2.2                                          i.e., Molding                                                                 Compound                                                                      (B-8)                                                                         Silicon   2.8-3.6        84      10.6                                         Copper    18             398     18                                           ______________________________________                                    

It is evident therefrom, that there is substantial mismatch of thermalcoefficients with respect to neighboring components. For example, thecoefficient of thermal expansion of silicon of the die 11 is at leastten time higher than that of and the plastic molding compound of thepackage 13, such being a poor thermal conductor. The result of themismatch is that the heat in the die 11 does not flow freely from thedie 11 through a thick mold compound and therefore thermal stress isinduced on the die 11.

Thermal stress on die 11 can cause varying levels of damage to the die.Because the die's ends 18 are subject to a much higher thermal stressduring the heating and cooling stages of the package mounting on ordemounting from the PC board, the die bends as shown in FIGS. 2A, 2B and2C. The die also experiences high thermal stress when the device/packageis going through thermal cycling continuously from peak loading tonormal loading or from power-down to power-up during actualdevice/package life operating conditions. During the heating, the dietends to expand, however, this expansion is prohibited because of themold compound and therefore, the thermal stress builds up. FIG. 2Arepresents the die's bending during the heating stage of processing.FIG. 2B portrays the die's configuration between heating and cooling,and FIG. 2C depicts the die's configuration during cooling. Thisrepeated bending due to extreme temperature cycling and power cyclingcan cause the die to crack or delaminate. The die either becomesimmediately non-functional or its life is shorten. In either event, thedie's bending causes undesirable damage.

OBJECTS AND SUMMARY OF THE INVENTION

in light of the aforementioned problems with the prior art, it istherefore an object of the present invention to provide a device andmethod for reducing thermal stress of an electrically-functional die.

It is another object of the present invention to provide a device andmethod for re-enforcing the strength of an electrically-functional diewithin a semiconductor package.

It is also an object to recycle defective and rejected dies inpracticing the present invention.

it is yet another object of the present invention to increase the testyield and therefore, the reliability of an electrically-functional diewithin a semiconductor package.

The foregoing and other objects of this invention are achieved byproviding, in a semiconductor package containing a die, anon-electrically functional, thermally conductive and substantiallyrigid member positioned adjacent to the die. The member can be arejected and defective die.

BRIEF DESCRIPTION OF THE DRAWINGS

This invention will be more clearly understood from the followingdescription when read in conjunction with the accompanying drawings, ofwhich:

FIG. 1 is a schematic diagram of a semiconductor package of the priorart;

FIGS. 2A-2C depicts a die on a die attachment pad of the prior artbending as they are subjected to a range of temperatures;

FIG. 3 shows a semiconductor package including a device of the presentinvention;

FIGS. 4A-4C depicts the device of the present invention positionedadjacent to a die which is on a die attachment pad, bending as they aresubjected to a range of temperatures;

FIG. 5 shows an alternative embodiment of the present invention;

FIG. 6 shows a second alternative embodiment of this invention; and

FIG. 7 shows yet another embodiment of the present invention.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

Referring to FIG. 3, a semiconductor package arrangement in accordancewith the present invention is shown. A plastic package 13 houses anelectrically-functional die 11 supported by a die attachment pad 12.Leads 16 provide electrical connection between the die 11 and an ICboard (not shown). A member 21 is positioned adjacent to the die 11 inaccordance with the present invention.

Member 21 is positioned so that it cooperates with die 11 to reducethermal stress of die 11. Member 21 is made of a material having acoefficient of thermal expansion which allows the heat in the die tomove across the junction of the common boundary of the die and themember 21, thereby allowing the heat to dissipate into member 21.Preferably, member 21 is made of a material having the same coefficientof thermal expansion as the die, and a much higher thermal conductivitythan the excapsulated material, thereby allowing the heat to spreadevenly. Therefore, because heat which in the prior art package remainsin the die 11 is now able to dissipate into the member 21, the thermalstress on the die 11 is reduced in accordance with the presentinvention.

In the present invention, the member also provides rigid support to thedie. Such support is not only beneficial during the temperaturevariations of the processing of the surface mount technology, but it isalso beneficial during the end use of the semiconductor device. Therigid support of the die in addition to that provided by the dieattachment pad 12 and the plastic package 13, further guarantees againstdamage to the die caused by vibration or shock. Furthermore, member 21protects the electrically-functional die against extraneous thermalstress and environmental damage.

Member 21 is bonded to the die in a manner which provides for thermalconductivity between the die and the member. However, a bonding materialwhich also provides for electrical conductivity, such as soft solder,should be avoided. Therefore, it is preferable to use anon-electrically-conductive adhesive such as epoxy to adhere the memberto the die. Bonding material layer 15, whilenon-electrically-conductive, it is thermally conductive. Therefore, thethermal mass, including that of die 11 and member 21, such available todissipate heat into package 13 is consolidated into a single heatdissipating structure.

FIGS. 4A-4C illustrate that while bending of the die 11 may not beentirely eliminated, it is reduced and therefore, thermal stress on thedie is reduced. FIGS. 2A-2C and FIGS. 4A-40 are positioned next to eachother to illustrate that during the heating phase of processing, thereis less bending in FIG. 4A than shown in FIG. 2A. FIGS. 4B and 2B whichdepict each configuration during the thermal cycling, that is, betweenheating and cooling, show that each configuration goes through at leasttwo contorting phases. By comparing FIG. 40 to FIG. 2C, it is evidentthat during the cooling phase of processing, there is less bending shownin FIG. 4C than shown in FIG. 2C. The reduction of bending during theheating and cooling phases provides for an end product less likely tosuffer from cracks and delamination.

In a preferred embodiment of this invention, member 21 is a defective orrejected die. In the manufacture of semiconductor products, there are ICchips produced which do not meet specifications. In the past, there wasno way to use these defective dies and therefore, they would bewastefully discarded. However, in accordance with the present invention,such defective dies are used as members 21 and therefore waste isavoided.

Furthermore, member 21 can be made from any suitable material such asany semiconductor, ceramic or dielectric material. In accordance withthe present invention, the material of the member has a coefficient ofthermal expansion similar to that of the die 11 or at least lower thanthat of the plastic used in package 13 so that heat can dissipate fromthe die 11 through member 21.

The configuration of the member positioned so that it is in cooperationwith the die within the semiconductor package can take many differentforms. FIGS. 5-7 are illustrative of this point. FIG. 5 shows aconfiguration where one electrically-functional die is positioned inthermal cooperation with two members, 21 and 22 on top of the die. FIG.6 shows a configuration where two members 21 and 22 are positioned inbetween the die attachment pad 12 and the die 11. FIG. 7 shows a the die11 sandwiched between member 21 and member 22. Other configurations canbe envisioned an are intended to be within the scope of this invention.

In the layered arrangements shown in FIGS. 5-7, a composite material isformed by joining the layers with bonding material layer 15, which isnon-electrically-conductive and thermally conductive. The layeredmaterials reinforce each other at each layer, and therefore the die 11.This is particularly important in the manufacturing process because thelayers act like a cushion and therefore protect against mold stressesduring the packaging process.

In FIGS. 3, 5 and 7, where the die is underneath one or more members 21and 22,, the top member serves as a shield to protect the functional die11 against moisture penetration. As illustrated by the arrows 24, inFIG. 5, to reach the die 11, the path that moisture travels is a longerdistance in this stacking die approach of the present invention than inthe single die approach. Accordingly, moisture related failure duringthe actual operation of the device is reduced.

What is claimed is:
 1. In a semiconductor package containing at leastone electrically-functional die having an active side to which aplurality of wires is attached, an arrangement comprising a dieattachment pad on a top of which is said electrically-functional die,and a non-electrically functional, thermally conductive andsubstantially rigid die having a common boundary with the active side ofthe electrically-functional die in a thermally transferable andphysically reinforcing position such that, said non-electricallyfunctional die reduces thermal stresses on said electrically-functionaldie and re-enforces the strength of said electrically-functional die,wherein said wires of the electrically-functional die are disposedbeyond the common boundary of said non-electrically functional die andsaid electrically-functional die.
 2. An arrangement as recited in Claim1 wherein said non- electrically functional die is a defective die. 3.An arrangement as recited in Claim 1 wherein said non-electricallyfunctional die and said electrically-functional die are arranged in amanner which reduces moisture precipitation at theelectrically-functional die.
 4. An arrangement as recited in claim 1wherein said arrangement comprises a plurality of non-electricallyfunctional dies on top of said electrically-functional die,
 5. Anarrangement as recited in claim 1 further comprising a thermallyconductive, non-electrically conductive bonding material disposedbetween said non-electrically function die and saidelectrically-functional die.
 6. The arrangement recited in claim 5wherein said bonding material is a non-electrically conductive epoxy. 7.In a semiconductor package containing at least oneelectrically-functional die having an electrically active side on whicha plurality of wires is disposed, said electrically-functional die beingdisposed on top of a die attachment pad, a method of reducing thermalstress on said electrically-functional die and for reinforcing thestrength of said electrically-functional die, said method comprising thestep of locating a non-electrically functional thermally conductive andsubstantially rigid die so as to have a common boundary with the activeside of said electrically-functional die in a thermally transferable andphysically reinforcing position such that the wires on said active sideof the electrically-functional die are disposed beyond the commonboundary of said electrically-functional die and said non-electricallyfunctional die.
 8. A method as recited in claim 7 further comprising thestep of: attaching said electrically-functional die to saidnon-electrically functional die with a thermally conductive,non-electrically conductive bonding material.
 9. A method as recited inclaim 1 wherein said non-electrically functional die is a defective die.